1. Field of the Invention
The present invention generally relates to improved devices within electric circuit breakers for avoiding unintended arc discharges which strike the external lug, and more particularly to devices which employ a plurality of spaced metal plates for dividing the arc into smaller segments. The present invention further relates to electric circuit breakers for service in intermediate size circuits having voltages of up to 600 volts, normal currents of up to 1,000 amperes and short circuit currents of from 10,000 to 200,000 amperes.
2. Description of the Prior Art
Circuit breakers in this intermediate range typically displace from 100 to 600 cubic inches. They must shut off within about 4 milliseconds in order to avoid serious internal damage. Because of heat and electromagnetic characteristics these circuit breakers use tin plated aluminum lugs which provide good connections to both copper and aluminum wire and avoid the hysteresis problems associated with steel lugs. Because of the relatively large currents involved, the contact arm of these circuit breakers is subjected to strong repulsive "blow off loop" forces under fault conditions. This requires that the contact arm be strong. Additional repulsive force, less than that created by "blow off", is provided by the rapid expansion of gases ionized by the arcing current. Vent holes in the casing of the breaker allow for dissipation of pressure from these expanding gases.
Circuit breakers operate through physical separation of contact members. During separation, an electric arc develops between the contact members. It is well known in the art for circuit breakers in the intermediate range to use a plurality of spaced apart metal plates (an "arc stack") to divide the electric arc into segments. Prior art circuit breakers typically orient the arc stack so that the plane of the metal plates cuts across the line of separation between the separating contact members. As the contact members continue to separate, the arc tends to move through the arc stack and may reignite on the opposite side of the arc stack and strike the lug directly through vent openings in the casing rather than through the intended path formed by the successive plates in the arc stack and thence through a conductor to the lug. This unintended lug striking may occur because the electric potential of plates near a separating contact member may be such that the arc finds it easier to communicate to the lug through a vent between plates rather than through successive arc plates. At the voltages and currents characteristic of circuits using these breakers, this reignition of the arc and unintended striking of the lug is likely to destroy the circuit breaker.
In prior art circuit breakers, a variety of techniques have been employed for suppressing the arc within the arc stack and avoiding unintended lug strikes, including barriers, insulators, and deflectors. These additional parts complicate the fabrication process and increase costs. It would be advantageous to reduce the parts required to avoid unintended lug strikes, and simplify the fabrication process.
Traditional production methods for contact arms, such as stamping or extrusion, tend to produce rectangular cross sections which in these intermediate circuit breakers can be more massive than necessary in order to achieve adequate strength.